German open application DE-OS 31 02 099 describes a lathe chuck whose chuck body has a plurality of clamp jaws which are controlled and moved radially by a drive member movable axially in the chuck body.
The drive member has its operation monitored by an operation-monitoring device acting on the rotary drive of the lathe chuck and which at least partially forms or is connected to a clamping piston which can be acted on by a pressurized medium to move it along an axial displacement path.
The operation-monotoring device comprises a sensor fixed in position outside the chunck, a switch member positioned movable on the outer circumference of the chuck body operating the sensor and a control piston moving the switch member which is acted on by a pressurized medium of the clamping piston against the force of a restoring spring and is lockable in a locked position disengaging the rotary drive from motion caused by the pressurized medium, if the drive member is outside a certain working region of its axial displacement.
The operation-monitoring device allows a control both of the clampling displacement and also the clamping force during operation of the chuck the rotation of the chuck is only possible when the displacement of a drive member required for secure clamping of the workpiece has taken place in the chuck and the clamping pressure has built up in the clamping piston to a sufficient degree.
In the known clamping device the control piston is guided radially in the chuck body and has a lock nose for its locking on its radial interior end which is associated with a locking cam on the drive member. This design for the control piston requires large radial dimensions in connection with the restoring spring acting on it which are often undesirable. Furthermore, the fact that the position of the control piston can be influenced by centrifugal forces can be troublesome.